Thank you for your help. I think I'm still having a problem with the line:

s/$pattern/qq{$replace}/eeg;

Here's the snippet that's failing:

# get strings from GUI.  
my $old = my $new = $_->children->get; # get old filename
my $pattern = $form->{'entry2'}->get_text();
# e.g. $pattern == "^(\d{2})\. (.*)$" after this
my $replace = $form->{'entry1'}->get_text();
# e.g. $replace == "two-$2, one-$1" after this

# is the following match necessary? $1, etc
# need to be assigned somehow...
if($old =~ /$pattern/) {
    $new =~ s/$pattern/qq{$replace}/eeg;
    print STDERR $old." => ".$new."\n";
}
[download]

After the $new =~ line, $new is empty, every time. Am I missing something? Thanks again